Annular barriers are used in wellbores for different purposes, such as for providing a barrier for flowing between an inner and an outer tubular structure or an inner tubular structure and an inner wall of the borehole. The annular barriers are mounted as part of the well tubular structure. An annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the inner wall of the annular barrier.
A second annular barrier is used to seal off a zone between an inner and an outer tubular structure or a well tubular structure and the borehole. The first annular barrier is expanded on one side of the zone to be sealed off, and the second annular barrier is expanded on the other side of that zone. Thus, the entire zone is sealed off.
The pressure envelope of a well is governed by the burst rating of the tubular and the well hardware etc. used within the well construction. In some circumstances, the expandable sleeve of an annular barrier may be expanded by increasing the pressure within the well, which is the most cost-efficient way of expanding the sleeve. The burst rating of a well defines the maximum pressure that can be applied to the well for expansion of the sleeve, and it is desirable to minimise the expansion pressure required for expanding the sleeve to minimise the exposure of the well to the expansion pressure.
When expanded, annular barriers may be subjected to a continuous differential pressure or a periodically high pressure within the annulus. One of the purposes of the barrier is to contain this differential pressure and prevent a leak across the barrier.
The ability of the expanded sleeve of an annular barrier to contain this pressure and seal against the wellbore (or outer pipe) is thus affected by many variables, such as strength of material, wall thickness, surface area exposed to the collapse pressure, temperature, well fluids, etc.
The ability to seal against the differential pressure within the annulus by the expanded sleeve in certain well environments is insufficient for some well applications. Thus, it is desirable to increase the ability to seal against the differential pressure within the annulus to enable use of annular barriers in all wells, specifically in wells that experience a high drawdown pressure during production and depletion. The ability to seal may be improved by increasing the wall thickness or the strength of the material or by changing the type of external elastomers mounted on the expansion sleeve. However, this would increase the expansion pressure, which is not desirable, as already mentioned.
It is thus desirable to provide a solution where the seal capability of the expanded sleeves is increased.